RNA labelled with tritium in all four nucleosides has been isolated from rabbit marrow erythroid cells and attempts to characterize it as mRNA will be made (a) by sequence analysis of large oligonucleotides from pancreatic ribonuclease digests (b) from binding studies to ribosomes and (c) its hybridization kinetics with DNA. Heterogeneous nuclear RNA from erythroid cells labelled with tritiated nucleosides after short pulse will be examined for sequences present in globin mRNA by direct sequence analysis after isolation of poly (A) containing fragments to determine the location of mRNA sequences at its 3' end, and also by its DNA hybridization properties in competition with mRNA. Analysis will also be made of RNA synthesized by RNA polymerase II from erythroid cells using erythroid cell chromatin as a template to determine if faithful synthesis of HnRNA is occurring.